This invention relates to an improved process for preparation of certain insecticidal esters of alpha-cyano-m-aryloxybenzyl alcohols known conventionally as synthetic pyrethroids. More particularly, this invention is directed to an improvement in the stepwise synthesis of such esters in which a distilled acid chloride intermediate is treated with an anhydrous hydrogen halide to obviate the formation of undesirable impurities in the final insecticidal product obtained on reaction of the acid chloride with a phenoxybenzaldehyde and a cyanide solution.
Esters of alpha-cyano-m-aryloxybenzyl alcohols are disclosed in U.S. Pat. No. 4,062,968 as being superior insecticides for certain economically important insects. In accordance with the disclosure in U.S. Pat. No. 4,056,509 the esters are prepared using 2-(4-chlorophenyl)-3-methyl butanoyloxy compounds, which in turn have been prepared via the hydrolysis of the alkylated benzyl cyanides also disclosed herein. Alternative methods for the preparation of the esters are disclosed in U.S. Pat. Nos. 4,110,360 to 4,110,363, inclusive.
One very suitable technique for preparing these esters proceeds stepwise through an acid chloride intermediate, said acid chloride being reacted with the appropriate phenoxybenzaldehyde in the presence of cyanide ion to yield the desired product. In this stepwise synthesis, the acid chloride intermediate is very conveniently prepared by reacting the corresponding acid with thionyl chloride. While this reaction affords high yields of the desired acid chloride intermediate, small quantities of by-products, e.g. heavy ends, are also produced which make it desirable to further purify this reaction product prior to reaction with the phenoxybenzaldehyde. For larger scale operations, fractional distillation is particularly suited for this purification step. In carrying out this distillation it has been found that while the acid chloride can be isolated in high yield and purity, there is formed and/or carried over from the chlorination, minor amounts of a ketene impurity which distills at the same conditions as the desired acid chloride. This ketene impurity is undesirable because it will subsequently react with the phenoxybenzaldehyde to produce a contaminant which is very difficult to remove from the final product. With the instant invention, a simple and effective technique has been devised to avoid formation of this undesirable ketene-derived impurity while retaining the benefits of the use of the distilled and purified acid chloride in the stepwise synthesis technique.
It is well known in the art that hydrogen halides add to ketenes to yield acid chlorides. However, until the present disclosure, the presence of a ketene as the cause of impurities in insecticidal esters prepared by reacting an acid chloride, a phenoxybenzaldehyde and a cyanide solution was unknown. Hydrogen chloride gas is used to treat p-thionylaminobenzoyl chloride to yield p-aminobenzoyl chloride hydrochloride in U.S. Pat. No. 3,597,475. Similarly, a p-aminobenzoic acid salt is prepared from p-aminobenzoic acid with anhydrous hydrogen chloride in U.S. Pat. No. 3,681,450. In a process for converting carboxylic acids to acid halides, Japanese Pat. No. 49,030,311 discloses the elimination of residual halogenating agent with water, monohydric or polyhydric alcohols, carboxylic acids or organic metal compounds.
The preparation of synthetic pyrethroids is disclosed in U.S. Pat. No. 4,062,968.